How Plasmodium malariae Defies Prevention Drugs
By Malaria Research Team | August 23, 2025
For decades, malaria researchers and healthcare providers have focused their attention on the most deadly form of malaria, Plasmodium falciparum, which claims hundreds of thousands of lives annually. Meanwhile, another malaria species has been quietly evolving a remarkable ability to evade our best preventive drugs—sometimes hiding undetected in the human body for decades before emerging to cause illness. Plasmodium malariae, one of the less famous malaria parasites, is rewriting our understanding of how these organisms survive and persist despite apparently effective chemoprophylaxis.
Recent research has uncovered a startling phenomenon: travelers who diligently take their malaria prevention medications still sometimes develop P. malariae infections months after returning home, without the genetic mutations we would expect to explain drug resistance.
This discovery challenges long-held assumptions about this parasite and suggests we may need to reconsider some fundamental aspects of malaria biology and prevention.
Plasmodium malariae differs from its more notorious relatives in several important ways. First described in the late 19th century, this parasite causes a form of malaria characterized by a distinctive 72-hour fever cycle (giving rise to the historical term "quartan malaria"), compared to the 48-hour cycle of P. vivax and P. falciparum. While generally less severe than falciparum malaria, P. malariae infections can persist for extraordinarily long periods—there are documented cases of symptoms appearing more than 40 years after initial exposure 1 .
Plasmodium malariae was first identified by Charles Louis Alphonse Laveran in 1880, the same physician who first discovered that malaria was caused by parasites.
What makes P. malariae particularly fascinating to scientists is its ability to maintain a low-level chronic infection that can fly under the radar of both our immune systems and routine diagnostic tests. Unlike P. vivax and P. ovale, which are known to form dormant liver stages (hypnozoites) that can reactivate months or years later, current medical dogma holds that P. malariae does not form hypnozoites. This conventional understanding makes its ability to persist despite drug treatment even more puzzling.
Though often overshadowed by its more deadly cousins, P. malariae has a wide geographic distribution across tropical regions of Africa, Asia, and South America. According to the World Malaria Report, it accounts for a small but significant percentage of malaria cases worldwide, though its true prevalence is likely underestimated due to diagnostic challenges 9 .
P. malariae is found in tropical regions across Africa, Southeast Asia, and South America, often coexisting with other malaria species.
Malaria chemoprophylaxis refers to the use of drugs to prevent malaria infection in people traveling to or living in endemic areas. Different medications target different stages of the parasite's life cycle:
Drugs that target the liver stage of infection, preventing the parasite from progressing to the blood stage
Drugs that target the blood stage, preventing clinical symptoms but not necessarily initial infection
Atovaquone-proguanil (marketed as Malarone) is a popular combination drug that offers both causal and suppressive prophylaxis. The atovaquone component disrupts the parasite's mitochondrial electron transport, while proguanil enhances this effect and also inhibits dihydrofolate reductase, an enzyme crucial for parasite reproduction 2 .
In malaria research, a curious pattern has emerged: travelers who take chemoprophylaxis are significantly more likely to report P. malariae infections compared to P. falciparum infections. One extensive case series found that out of 378 evaluable P. malariae cases, 100 (26.2%) reported using at least partial chemoprophylaxis. This pattern resembles what is seen with the relapsing parasites P. ovale and P. vivax, but stands in stark contrast to imported P. falciparum cases, where only 7.5% reported any chemoprophylaxis use 1 3 .
This pattern suggests that P. malariae might have special abilities to evade chemoprophylaxis that we don't fully understand—a possibility that has significant implications for malaria control and prevention strategies.
To investigate why P. malariae seemed to be breaking through chemoprophylaxis at unexpectedly high rates, an international team of researchers conducted a comprehensive analysis of imported malaria cases. They assembled an extensive case series of P. malariae patients presenting in non-endemic countries (China, Sweden, and the UK) who had returned from travel in endemic countries, primarily in Africa 3 .
The research team performed genetic analysis on parasites from patients who reported use of atovaquone-proguanil chemoprophylaxis. Specifically, they looked for mutations at codon 268 of the cytb gene in the P. malariae mitochondrial genome (pmcytb). This location was chosen because in P. falciparum, mutations at the corresponding position (Tyr268Ser, Tyr268Asn, or Tyr268Cys) are known to be associated with resistance to atovaquone-proguanil 8 .
The results were surprising. Despite the patients having taken atovaquone-proguanil and still developing P. malariae infection, none of the parasite samples showed mutations at codon 268 of pmcytb 1 3 . This finding suggests that the mechanism behind these prophylaxis breakthroughs is different from the known resistance mechanisms in P. falciparum.
| Plasmodium Species | Total Cases | Cases Reporting Chemoprophylaxis Use | Percentage |
|---|---|---|---|
| P. malariae | 378 | 100 | 26.2% |
| P. falciparum | Not specified | Not specified | 7.5%* |
| P. ovale spp. | Similar pattern to P. malariae | ||
| P. vivax | Similar pattern to P. malariae | ||
The researchers also examined the latency periods—the time between potential exposure and the onset of symptoms—for P. malariae infections. They found that chemoprophylaxis use was associated with significantly delayed onset of symptoms. This pattern has been observed with other malaria species as well; one study of travelers found that chemoprophylaxis use delayed symptom onset in P. vivax and P. ovale infections by a median of 46 and 67 days respectively, compared to 12 and 15 days in those not taking prophylaxis .
| Plasmodium Species | Median Latency with Chemoprophylaxis | Median Latency without Chemoprophylaxis |
|---|---|---|
| P. vivax | 46 days | 12 days |
| P. ovale | 67 days | 15 days |
| P. malariae | Similar pattern likely (exact figures not provided in search results) | |
Understanding how researchers study malaria drug resistance helps appreciate the significance of these findings. Here are some of the key tools and methods used in this field:
| Tool/Reagent | Function/Application | Example in P. malariae Research |
|---|---|---|
| PCR-RFLP | Detects specific genetic mutations through DNA amplification and enzyme digestion | Used to screen for mutations at codon 268 of pmcytb 8 |
| Mitochondrial DNA sequencing | Allows comprehensive analysis of genetic changes in parasite mitochondrial genome | Confirmed absence of mutations at resistance-associated positions 3 |
| Blood sample collection filters | Preserves blood samples for DNA analysis without refrigeration | Enabled collection of samples from diverse field locations 8 |
| Species-specific PCR primers | Amplifies DNA from specific malaria species | Distinguished P. malariae from other Plasmodium species 3 |
| Restriction enzymes (NsiI, AlwNI, SspI) | Cut DNA at specific sequences to identify mutations | Used in RFLP analysis to detect cytb mutations 8 |
| N-Benzyl-N-Methylacetoacetamide | 71392-09-1 | C12H15NO2 |
| 2-(3-Chlorophenyl)butanoic acid | 188014-55-3 | C10H11ClO2 |
| Diselenide, bis(1-methylethyl)- | 37826-18-9 | C6H14Se2 |
| 1,2,3-Tris(fluoromethyl)benzene | 921595-54-2 | C9H9F3 |
| L-beta-aspartyl-L-aspartic acid | 60079-22-3 | C8H12N2O7 |
Advanced genetic techniques allow researchers to identify even minute mutations in parasite DNA that might confer drug resistance.
Maintaining parasite cultures in the laboratory enables researchers to test drug susceptibility under controlled conditions.
The most intriguing implication of this research is the possibility that P. malariae might have a latent liver stage similar to the hypnozoites of P. vivax and P. ovale, despite current scientific consensus that it does not 1 3 7 . The study authors propose that "P. malariae can initiate a latent hypnozoite developmental programme in the human hepatocyte"—a hypothesis that, if validated, would explain the consistent observations of remarkable longevity of parasitism, even in the presence of antimalarial prophylaxis or treatment.
This would represent a paradigm shift in our understanding of P. malariae biology and would have significant implications for malaria treatment and prevention strategies. If P. malariae does form hypnozoites, we might need to consider using radical cure medications (like primaquine or tafenoquine) for this species as well.
For travelers and healthcare providers, these findings highlight several important considerations:
The delayed onset of symptoms in chemoprophylaxis users means that travelers need to be aware that malaria symptoms can appear weeks or months after return from endemic areas, and should seek immediate medical attention if symptoms develop.
Since P. malariae often presents with low parasite densities, it can be easily missed by routine microscopy. Molecular diagnostic methods (PCR) may be necessary for accurate detection 9 .
The effectiveness of chemoprophylaxis depends heavily on proper adherence to the medication regimen. Travelers should be counseled on the importance of taking medications as prescribed, both during travel and for the recommended period after return 4 .
This research opens several promising avenues for future investigation:
More extensive genetic analysis of P. malariae isolates from different geographic regions may identify other mechanisms of drug tolerance or resistance.
Developing better in vitro and animal models to study the potential liver stages of P. malariae could confirm or refute the hypnozoite hypothesis.
Evaluating the effectiveness of radical cure medications against P. malariae could provide indirect evidence of hypnozoite existence.
Improving surveillance for P. malariae in endemic areas would give us a better understanding of its true prevalence and distribution.
The story of Plasmodium malariae's ability to break through chemoprophylaxis without the expected genetic mutations reminds us that nature often defies our neat categorizations. What we thought we knew about malaria biology continues to be challenged by observant clinicians and diligent researchers connecting patterns across patient cases and laboratory findings.
This research underscores the importance of studying all malaria species, not just the most common or deadly ones. By understanding the unusual capabilities of P. malariae, we may uncover biological mechanisms that could inform new approaches to malaria prevention and treatment more broadly.
For now, travelers to malaria-endemic areas should continue to take recommended chemoprophylaxis and mosquito protection measures, as these remain highly effective against the most dangerous forms of malaria. However, they should also be aware that delayed symptoms are possible even after apparently successful prophylaxis, and should seek medical attention if fever develops after return—even months later.
As science continues to unravel the mysteries of this stealthy parasite, we move closer to a more comprehensive understanding of malaria and better tools to combat all its forms.